This invention relates generally to variable capacitance means and more particularly to variable capacitance means fabricated in integrated circuits to adjust the frequency of a highly accurate temperature compensated oscillator, such as quartz or crystal oscillator which have many different kinds of applications, such as in communication systems, e.g., pagers, mobile subscriber sets or the like.
The frequency operation of crystal oscillators or resonators is subjected to variation to due to environmental conditions, particularly temperature, and due to aging. The operating frequency of the oscillator can be changed to compensate for such changes by the use of load capacitances placed in series with the oscillator. The frequency excursion experienced by the oscillator due to temperature changes depends on the type of oscillator and the value of the series load capacitance.
Many different approaches have been devised for providing temperature compensation of these oscillators, and a substantial number comprise means for measuring temperature excursion of the oscillator for a given load capacitance and compensating for the excursion by adjusting the value of the load capacitance to compensate for changes in frequency induced by ambient temperature changes so that the frequency of operation is returned to a predetermined value thereby stabilizing oscillation frequency. Laid-open patent application (Tokkaisho) 62-76801 to H. Nishihara, published Apr. 8, 1987, is one example that fairly represents the present state of the art wherein there is shown in FIG. 1 a variable capacitance array coupled in series with a crystal oscillator wherein the individual capacitor elements are each controlled by means of FET switches. An output, representing a temperature compensating value based upon the detected ambient temperature of the oscillator, is provided from a memory, such as a look-up table, and converted from series to parallel format and applied to the FET switches to selectively switch in and out capacitor elements in the capacitor array for selected connection thereof in series with the oscillator. The output is provided with a time constant by a charge/discharge circuit after which the capacitor switching takes place. The purpose of compensation is to improve the accuracy of a stabilized frequency in the oscillator output, i.e., the C/N ratio. However, since the oscillator drain capacitance is digitally changed, the oscillation frequency is changed, i.e., increased or decreased, abruptly so that fine frequency stabilization is not achieved. Furthermore, it has been difficult to date to fabricate charge/discharge circuits having sufficiently large time constants in semiconductor integrated circuit form.
In addition, because of the high level of accuracy required, it is necessary that each individual capacitance element employed in the capacitor array be of relatively small value. With an increased number in capacitor elements in the switched capacitor array, the capacitor elements are parasitic relative to FET switching capacitance, e.g., the source-drain capacitance or total impedance, reached a total capacitance value in the capacitor array that can not be ignored, and, as a result, the capacitance ratio of the on and off times of the capacitor array is small. Thus, adjustment of the oscillating frequency of the oscillator over a broad range becomes not possible. In addition, if the number of capacitor elements in the array is increased, the compensating memory capacity required to accommodate the increased capacitor element capacity translates into greater costs in manufacture and production of solid state temperature compensated oscillator integrated circuits.
Thus, it is an object of this invention to provide a temperature compensated crystal oscillator with improved temperature compensation resulting in enhanced frequency stability, which is highly reliable for use in communication systems.
It is another object of this invention to provide a variable capacitance means in a fully integrated circuit form that provides for smooth, fine incremental transitional change in equivalent capacitance relative to such means.
It is another object of this invention to provide a variable capacitance means that has improved fine adjustment capabilities for oscillation frequency adjustment of a crystal oscillator requiring a reduced memory capacity required for storing temperature compensation values requiring no increase in the number of discrete capacitance components in the capacitor circuit array of the variable capacitance means.
It is another object of this invention to offer a variable capacitance means that requires less switching, provides a large capacitance on/off ratio, and allows for fine adjustments to be made utilizing a small amount of memory capacity for required temperature compensating values.